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doc: Explain "file-like objects".

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* doc/guix.texi (G-Expressions): Mention "file-like objects" and explain
  more.
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Ludovic Courtès 2015-06-05 14:53:32 +02:00
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@ -2942,12 +2942,12 @@ and these dependencies are automatically added as inputs to the build
processes that use them.
@end itemize
Actually this mechanism is not limited to package and derivation
objects; @dfn{compilers} able to ``lower'' other high-level objects to
This mechanism is not limited to package and derivation
objects: @dfn{compilers} able to ``lower'' other high-level objects to
derivations can be defined, such that these objects can also be inserted
into gexps. Another useful type of high-level object that can be
inserted in a gexp is @dfn{local files}, which allows files from the
local file system to be added to the store and referred to by
into gexps. For example, a useful type of high-level object that can be
inserted in a gexp is ``file-like objects'', which make it easy to
add files to the store and refer to them in
derivations and such (see @code{local-file} and @code{plain-file}
below.)
@ -3113,6 +3113,24 @@ refer to. Any reference to another store item will lead to a build error.
The other arguments are as for @code{derivation} (@pxref{Derivations}).
@end deffn
@cindex file-like objects
The @code{local-file} and @code{plain-file} procedures below return
@dfn{file-like objects}. That is, when unquoted in a G-expression,
these objects lead to a file in the store. Consider this G-expression:
@example
#~(system* (string-append #$glibc "/sbin/nscd") "-f"
#$(local-file "/tmp/my-nscd.conf"))
@end example
The effect here is to ``intern'' @file{/tmp/my-nscd.conf} by copying it
to the store. Once expanded, for instance @i{via}
@code{gexp->derivation}, the G-expression refers to that copy under
@file{/gnu/store}; thus, modifying or removing the file in @file{/tmp}
does not have any effect on what the G-expression does.
@code{plain-file} can be used similarly; it differs in that the file
content is directly passed as a string.
@deffn {Scheme Procedure} local-file @var{file} [@var{name}] @
[#:recursive? #t]
Return an object representing local file @var{file} to add to the store; this